In a skillet, melt 2 tablespoons of the butter. Add the leek and cook over moderate heat, stirring, until softened and lightly browned, 8 minutes. Stir in the vinegar, transfer to a bowl and let cool. Stir in the garlic, lemon juice and Tabasco. Soften the remaining 4 tablespoons of butter and combine thoroughly with the leek mixture. Season with salt.